How to make just one color of my texture bright (like as metallic)?
You can use color to determine your mentalist mask:



I use a distance vector math node, which tell blender to find the color in that image, with RGB (1.0, 1.0 ,0.0)(which is yellow). 

And then continue with a compare node to see if color is yellow with some ambiguity. 

(the second value is compared value, set to 0.0 means same color. the third one is tolerance, change it yourself to see the effect)

Using this mask, you can do a lot of thing with it, you can change the metallic value using this mask and invert it to make it look like mirror in roughness setting.

More complicated setting: